1. Field of the Invention
The present invention relates to splittable optical fiber ribbons and, in particular, optical fiber ribbons for telecommunications cables. More particularly, the present invention is directed to the interface between adjacent sub-unit ribbons within splittable double-embedded optical fiber ribbons.
2. Related Art
Related-art double-embedded optical fiber ribbons are designed with sub-unit having rounded edges. Each sub-unit is formed of a plurality of optical fibers disposed in a plane and encapsulated with a polymer thereby forming the sub-unit. Each sub-unit includes rounded edges, which are placed adjacent one another, when the sub-units are arranged side-by-side so that the optical fibers of all the sub-units lie substantially in a plane. The sub-units are then encapsulated by a second polymer coat to form one optical fiber ribbon therefrom.
A related-art sub-unit design, and ribbon made therefrom, are shown in FIG. 1. A first sub-unit 1xe2x80x2 includes optical fibers 3xe2x80x2 encapsulated within a matrix material 4xe2x80x2. Similarly, a second sub-unit 2xe2x80x2 includes optical fibers 3xe2x80x2 encapsulated within a matrix material 4xe2x80x2. Both of the first and second sub-units 1xe2x80x2 and 2xe2x80x2 have rounded edges.
In the related-art design, however, alignment of the sub-units so that all the optical fibers remain in a plane as the second polymer coat cures is a problem, i.e., planarity of the ribbon formed by the sub-units is poor. The sub-units 1xe2x80x2 and 2xe2x80x2 include rounded edges that contact one another generally at a point 6xe2x80x2, i.e., as two half circles would contact. Because of the point contact, the sub-units 1xe2x80x2 and 2xe2x80x2 tend to rotate relative to one another about the contact point which thereby causes poor ribbon planarity. Yet good ribbon planarity is an important ribbon property. Ribbon planarity is a measure of optical fiber alignment in a ribbon and is a measure of ribbon quality. The ribbon planarity plays an important part in fusion and mechanical multi-fiber splicing, as well as in termination in the field. Ribbon planarity is measured according to industry standards wherein a planarity of zero indicates that all fibers in the ribbon lie in the same plane. Therefore, it is desirable to achieve a ribbon planarity which is as low as possible.
FIG. 1 shows a ribbon having poorxe2x80x94i.e., highxe2x80x94planarity. The rotation of the two sub-unit ribbons is due, at least in part, to the surface tension effects occurring in the second matrix material as it is cured between or among sub-unit ribbons during the curing process. In addition, rotation of the sub-unit ribbons may be due to the fact that different ribbon sub-units may be under different pay-off and take-up tensions during the ribbonizing process. Thirdly, the sub-unit ribbons may have electrostatic forces and may be under vibration during ribbon processing due to the instability of the equipment. These factors may lead toxe2x80x94due to the point-contact between sub- unitsxe2x80x94a higher or inconsistent ribbon planarity. For instance, the planarity may be greater than 50 xcexcm, which is too large for certain specificationsxe2x80x94US GR-20-CORE, issue 2, Italy""s Telecom Italia, and Swedish Telia, for examplexe2x80x94depending on the fiber count in the ribbon.
The ribbon sub-units include a minimum of two fibers and can include up to 12-fibers per sub-unit. A ribbon includes of a minimum of two ribbon sub-units and each sub-unit may include a different number of optical fibers. For example, a ribbon may have three sub-units , wherein each sub-unit may include either 2, or 4, or 6, or 8, or 12, or any other number of, optical fibers.
An object of the present invention is to overcome the problems of the related art. A further object of the present invention is to alleviate, or at the very least, minimize the related-art problem of poorxe2x80x94i.e., highxe2x80x94planarity in double-embedded optical fiber ribbons formed from sub-unit ribbons.
Planarity in a double-embedded optical fiber ribbon is improvedxe2x80x94i.e., loweredxe2x80x94in the present invention, by enhancing the stability of the interface between the sub-unit ribbons from which the double-embedded optical fiber ribbon is made. The present invention enhances interface stability by shaping the sub-unit-ribbon edges so as to create an interlocking effect and a greater contact area between the sub-units. The interlocking effect and greater contact area reduce sub-unit-ribbon interfacial micro-movement during formation of the double-embedded optical fiber ribbon.
Interface stability may be enhanced by making the sub-unit ribbon edges of a complementary shape so as to create an interlocking effect. For example, the edges may be complimentarily shaped by inclusion of a non-circular surface on one edge of each sub-unit. The non-circular surfaces, when adjacent one another, reduce rotation or rolling motion of the sub-units about the interface, i.e., the non-circular surfaces tend to interlock the sub-unit ribbons. The edges of the sub-units, opposite those edges which include a non-circular surface, may be either rounded or may also include a non-circular surface.
A sub-unit ribbon wherein both edges include a non-circular surface is advantageous when producing a double-embedded optical fiber ribbon of more than two sub-units. That is, the double-embedded optical fiber ribbon may be made of any number of sub-units so that it includes the desired number of optical fibers. A sub-unit having only one edge that includes a non-circular surface is advantageously disposed on the edges of the double-embedded optical fiber ribbon. That is, a sub-unit having one rounded edge and one edge that includes a non-circular surface can be arranged so that its rounded edge is disposed toward the outer edge of the double-embedded optical fiber, whereas the edge having a non-circular surface is disposed adjacent another sub-unit""s non-circular surface.
The non-circular surfaces may be included in variously shaped sub-unit edges, all of which enhance the stability of the interface between the sub-units by resisting relative rotation. The non-circular surfaces may be substantially perpendicular to the plane in which the sub-unit""s optical fibers lie. Alternatively, the non-circular surface may be oblique to the plane in which the sub-unit""s optical fibers lie. Further still, the non-circular surfaces may include protrusions extending therefrom. Not only do sub-units that have a non-circular surface on their edges enhance resistance to rotation, they are also complementarily shaped so as to provide an increased contact area between the sub-unit edges.
Instead of having edges with circular surfaces that point-contact one another, the stability of the interface between sub-units can be increased using rounded edges that have an increased contact area therebetween. That is, one sub-unit may have an edge with a concave circular surface, whereas an adjacent sub-unit may then have a convex circular surface which fits within the concave circular surface. Thus, instead of contacting at a point as do the circular or rounded edges in the related art, the circular or rounded edges of the present invention sub-units contact one another over a large portion of their complementarily rounded edges.
With each of the edge configurations of the present invention, the sub-units may be separated by the second matrix material, or may directly abut one another.
Not only do the above mentioned edge shapes provide an interlocking effect, they also provide a greater contact area between the sub-units, thereby further enhancing stability of the sub-unit interface. When the sub-units are separated by the second matrix material, a greater opposing surface area for limiting sub-unit rotation is achieved by the above-mentioned edge shapes. Further, when the sub-units are directly abutted, without any of the second matrix material therebetween, they are even less likely to rotate relative to one another.
Further, when there is no second polymer coat between the sub-units, not only is planarity improvedxe2x80x94i.e., loweredxe2x80x94and less second matrix material required but, also, the sub-units are more easily separated. That is, split-ability of the ribbon is enhanced because the sub-units are allowed to slide relative to one another along their contact interface when a splitting force is applied in a direction parallel to the contact surface. Although split-ability is enhanced, the sub-units still provide an increased resistance to relative rotation because the contact interface is less susceptible to surface tension forces arising from the outer matrix material during the curing process, and is less susceptible to other factors which cause relative rotation.
The sub-unit ribbons may be formed using any known method. In one method, for example, individual optical fibers are aligned in a common plane and fed through a die in which a matrix material is applied to form the sub-unit ribbon structure. The material for the matrix material in each sub-unit ribbon may be any of the conventionally used matrix materials. One well-known matrix material used for optical fiber ribbons is a polyurethane acrylate resin. Such a resin material may be ultraviolet (UV) curable, so that after the optical fibers pass through a die and resin material is applied to form the matrix material over the optical fibers, each entire sub-unit structure is passed through a UV light source for curing the resin. It has been found that such a sub-unit ribbon structure provides for secure retention of individual optical fibers therein due to the properties of the matrix material itself, as well as due to a strong and durable bond between the matrix material and the optical fibers. More specifically, the matrix material is selected from a group of materials which is strong, elastic, flexible, resistant to heat and other elements, etc. For example, the matrix material may be a UV curable polyurethane acrylate resin as mentioned above, and as manufactured by DSM Desotech inc., Elgin, Ill., USA. Alternatively, other commercially available resin materials such as DSM 900 series or BORDEN 100 series, for example, may be used as the matrix material, as is known to those skilled in the art. Additionally, thermoplastic materials such as polypropylene or polyamide, or copolymers, for example, may be used as the matrix material. As is known to those skilled in the art, once the thermoplastic material is applied as the matrix material in a molten state, it is solidified by the removal of heat.
Further, the second matrix material of the double-embedded optical fiber ribbon may be any of the conventionally used matrix materials and may be the same as, or different from, the material used for the matrix material in the sub-unit ribbons.
Although the sub-unit ribbons may be made by any known method, the present invention contemplates a specially shaped die for making multiple sub-unit ribbons at one time, which sub-units are already properly aligned for encapsulation by a second matrix material. That is, it is preferable to make the sub-units and form them into a double-embedded optical fiber ribbon in a one-step process.